It is known in the art to use inks, compositions or layers containing magnetic or magnetizable particles or pigments, particularly also magnetic optically variable pigments, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetizable particles are disclosed for example in U.S. Pat. Nos. 2,570,856; 3,676,273; 3,791,864; 5,630,877 and 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features one the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a therewith secured document or item, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.
A particularly striking optical effect can be achieved if a security feature changes its appearance in view to a change in viewing conditions, such as the viewing angle. Such an effect can e.g. by obtained by dynamic appearance-changing optical devices (DACODs), such as concave, respectively convex Fresnel type reflecting surfaces relying on oriented pigment particles in a hardened coating layer, as disclosed in EP 1 710 756 A1. This document describes one way to obtain a printed image that contains pigment particles or flakes having magnetic properties by aligning the pigment particles in a magnetic field. The pigment particles or flakes, after their alignment in a magnetic field, show a Fresnel structure arrangement, such as a Fresnel reflector. By tilting the image and thereby changing the direction of reflection towards a viewer, the area showing the greatest reflection to the viewer moves according to the alignment of the flakes or pigment particles (FIG. 1).
While the Fresnel type reflecting surfaces are flat, they can be made to provide the appearance of a concave or convex reflecting curved surface such as e.g. a cylinder or a hemisphere. Said Fresnel type reflecting surfaces can be produced by exposing a wet coating layer comprising non-isotropically reflecting magnetic or magnetizable pigment particles to the magnetic field of a single dipole magnet, wherein the latter is disposed above for concave effect (FIGS. 2B and 2C bottom), respectively below the plane of the coating layer for convex effect (FIGS. 2A and 2C top), as illustrated in FIG. 7B of EP 1 710 756 A1 for a convex orientation. The so-oriented pigment particles are consequently fixed/frozen in position and orientation by hardening the coating layer.
One example of such a structure is the so-called “rolling bar” effect, as disclosed in US 2005/0106367. A “rolling bar” effect is based on pigment particles orientation imitating a curved surface across the coating. The observer sees a specular reflection zone which moves away or towards the observer as the image is tilted. A so-called positive rolling bar comprises pigment particles oriented in a concave fashion (FIG. 2B) and follows a positively curved surface; a positive rolling bar moves with the rotation sense of tilting. A so-called negative rolling bar comprises pigment particles oriented in a convex fashion (FIGS. 1 and 2A) and follows a negatively curved surface; a negative rolling bar moves against the rotation sense of tilting. A hardened coating comprising pigment particles having an orientation following a concave curvature (positive curve orientation), shows a visual effect characterized by an upward movement of the rolling bar (positive rolling bar) when the support is tilted backwards. The concave curvature refers to the curvature as seen by an observer viewing the hardened coating from the side of the support carrying the hardened coating (FIG. 2B). A hardened coating comprising pigment particles having an orientation following a convex curvature (negative curve orientation, FIG. 2A) shows a visual effect characterized by a downward movement of the rolling bar (negative rolling bar) when the support carrying the hardened coating is tilted backwards (i.e. the top of the support moves away from the observer while the bottom of the support moves towards from the observer) (FIG. 1). This effect is nowadays utilized for a number of security elements on banknotes, such as on the “5” and the“10” of the 5 respectively 10 Euro banknote or the “100” of the 100 Rand banknote of South Africa.
For optical effect layers printed on a substrate, negative rolling bar effect (orientation of the pigment particles (220) in a convex fashion, curve of FIG. 2A) are produced by exposing a wet coating layer to the magnetic field of a magnet disposed on the opposite side of the substrate to the coating layer (FIG. 2C top), while positive rolling bar effect (orientation of the pigment particles (220) in a concave fashion, curve of FIG. 2B) are produced by exposing a wet coating layer to the magnetic field of a magnet disposed on the same side of the substrate as the coating layer (FIG. 2C bottom). Examples of positive and negative rolling bar effect and combinations thereof have been disclosed in US 2005/0106367 and in WO 2012/104098 A1. For positive rolling bar, the position of the magnet facing the still wet coating layer prevents the simultaneous curing of the coating layer with a UV irradiation source facing the coating layer.
U.S. Pat. No. 2,829,862 teaches the importance of the viscoelastic properties of the carrier material for preventing reorientation of the magnetic particles after the removal of the external magnet. Keeping the coating composition comprising the magnetic or magnetizable pigment particles or flakes within the magnetic field during the hardening process can preserve the orientation of the magnetic or magnetizable pigment particles or flakes Examples of such processes (as illustrated FIG. 3A) are disclosed for example in WO 2012/038531, EP 2433798 A1 or in US 2005/0106367A1. In all these examples, the external magnetic device is located on the side of the substrate opposite to the side carrying the coating composition and the hardening process is triggered by an irradiation source positioned on the side of the substrate carrying the coating composition.
The co-pending application EP 14178901.6 discloses a method for producing image coated articles by using magnetic pigments. The method comprises the steps of i) applying to a substrate a coating composition comprising a plurality of magnetic or magnetizable pigment particles, ii) exposing the coating layer to the magnetic field of a magnetic-field-generating device and iii) simultaneously or partially simultaneously hardening the coating layer through the substrate the coating layer the with a UV-Vis radiation source. The magnetic-field-generating device disclosed in EP 14178901.6 is located on the side of the substrate carrying the coating layer and the hardening process is triggered by UV-Vis radiation source positioned on the side of the substrate opposite to the side carrying the coating, i.e. hardening is carried out through the substrate.
WO 02/090002 A2 discloses a method for producing images on coated articles. The method (as illustrated in FIG. 4) comprises the steps of i) applying a layer of magnetizable pigment coating in liquid form on a substrate, with the magnetizable pigment coating containing a plurality of magnetic non-spherical particles or flakes, ii) exposing the pigment coating to a magnetic field and iii) solidifying the pigment coating by exposure to electromagnetic radiation. During the solidifying step, an external photomask with voids may be positioned between the pigment coating and the electromagnetic radiation source. The photomask disclosed in WO 02/090002 A2, allows to solidify only the exposed regions of the pigment coating facing the voids of the photomask thereby allowing the orientation of the flakes to be fixed/frozen only in those regions. The flakes dispersed in the un-exposed parts of the pigment coating may be re-oriented, in a subsequent step, using a second magnetic field. The pattern formed by the selective solidifying with a photomask allows for a higher resolution imaging than can be obtained by use of patterned magnetic fields or for patterns that cannot be achieved with simple magnetic fields. In this process, it is mandatory to keep the relative position of the coated substrate and the photomask constant during the solidifying step. As a consequence, the coated substrate may not be moved in a continuous translation movement in front of a fixed photomask and electromagnetic radiation source.
Therefore there is a need for a process for producing optical effect layers involving a photomask that would move in an absolutely concomitant mode as the applied coating comprising magnetic or magnetizable pigment particles. In particular, there is a need for producing optical effect layers comprising a motif made of at least two areas having different magnetic or magnetizable pigment particles orientation patterns in an efficient manner, with a high resolution and exact register.